During operation of a process power supply, various limit values, (for example, for current, voltage, power), are monitored in order to promote secure and optimal operation. For instance, target values for a forward power, a reverse power and a power at load are specified in the case of a power control of a process power supply. In order to ensure that the corresponding target values are not exceeded, the corresponding variables are controlled by varying an adjustment value by means of a control circuit, in such a manner that, as far as possible, all of the target values are undercut by the actual values. For this purpose, the control difference (a difference between target value and actual value) is formed for every channel. A channel selection selects a control channel as the active channel. The control channel selected as the active channel provides the control difference for input into the downstream controller. The adjustment value is then varied by the downstream controller in such a manner that the control difference of the active channel is minimized. Selection of the active control channel is implemented on the basis of the previously determined control difference.
U.S. Pat. No. 7,206,210 discloses 4 selecting a control channel that comprises the smallest or respectively the largest control difference (error signal). However, selecting the control channel with the largest control difference is extremely risky because the target values are routinely exceeded. Outside of the framework of settling time, selecting the largest control difference should generally be avoided.
Accordingly, it is therefore conventional to select the control channel having the smallest control difference as the basis for controlling a control loop. However, if an actual value of one of the control channels overshoots a target value, the control is implemented on the control channel on which the actual value has overshot the target value, because the associated control difference is negative and is therefore the smallest control difference. The overshooting of the target value is therefore corrected again in the next control step. By contrast, if the control channel with the largest control difference were to be selected as the active channel, the overshooting of the target value would continue to be ignored.
However, there are systems in which two target values are disposed very far apart. For example, in the case of a power control of a process power supply, the target values of the forward power and the reverse power are often disposed far apart. With the method known from the prior art, the control is often implemented, in such cases, over a long duration of the settling time, exclusively on the smallest target value, because its absolute control difference over the entire settling time is also the smallest. However, as a consequence, this has a long settling time (the time which the system requires in order to increase the output power from 10% of the adjusted target value to 90% of the adjusted target value), because the smaller the control difference for the control itself, the longer the settling time will be.